1. Technical Field of the Invention
The present invention relates generally to a fuel supply system which may be employed in automotive common rail fuel injection systems, and more particularly to such a fuel supply system which is equipped with a fuel filter installed downstream of a feed pump and designed to have a simple structure which ensures the mountability thereof in vehicles and may be produced at a low cost.
2. Background Art
Typical fuel supply systems for use in accumulator fuel injection systems for diesel engines are equipped with a high-pressure pump, a feed pump, a suction control valve (i.e., a flow rate control valve), and a fuel filter. The high-pressure pump works to pressurize and deliver fuel to a common rail in which the fuel is accumulated at a controlled high pressure. The feed pump works to pump the fuel out of a fuel tank and feed it to the high-pressure pump. The suction control valve works to control the flow rate of the fuel to be fed from the feed pump to the high-pressure pump. The fuel filter is equipped with a filter medium to filter the fuel. The capturing of smaller foreign objects is achieved by decreasing the mesh size of the filter medium. This, however, gives rise to the problem of increasing a loss of the pressure of fuel passing through the fuel filter and also results in an increased possibility of clogging of the fuel filter. The fuel usually becomes wax-like at low temperatures, thus resulting in an increased loss of the pressure of the fuel passing through the fuel filter, which leads to decreased performance or failure in operation of the feed pump.
In order to avoid the above drawbacks, Japanese Patent First Publication No. 2006-207499 teaches a fuel supply system designed to have the fuel filter disposed downstream of the feed pump to develop a greater difference in pressure across the fuel filter than when the fuel filter is disposed upstream of the feed pump. This allows the mesh size of the filter medium to be decreased to improve the ability of the fuel filter to trap foreign objects and also minimizes the deterioration in performance or the failure in operation of the feed pump when the fuel filter is clogged or the fuel becomes wax-like at the low temperatures.
The fuel supply system in which the fuel filter is disposed downstream of the feed pump, however, encounters the drawback in that a total production cost is increased due to two factors, as discussed below.
The first is the need for disposing two valves: one upstream and the other downstream of the fuel filter. Specifically, a pressure control valve is used to stabilize or keep the pressure of fuel between the fuel filter and the suction control valve at a set level in order to ensure the accuracy in controlling the flow rate of the fuel through the suction control valve. A relief valve is used to return an excess of the fuel to upstream of the feed pump to control the flow rate of the fuel passing through the fuel filter in order to avoid the breakage or early clogging of the filter medium.
The second is associated with the fuel priming after the engine is installed in the vehicle. Specifically, after the fuel supply system is joined to the engine, a fuel pipe between the fuel tank and the feed pump and the fuel filter usually need to be filled with fuel in order to ensure the stability in starting the engine. It is easy for the fuel supply pump in which the fuel filter is disposed downstream of the feed pump to fill the fuel pipe with the fuel between the fuel tank and the feed pump, but however, it is difficult to fill the fuel filter with the fuel because the feed pump installed upstream of the fuel filter is small in area of an internal fuel path thereof. The fuel supply system is, therefore, designed to have a bypass path extending directly to the fuel filter to fill the fuel filter with the fuel and a check valve installed in the bypass path, which leads to the increase in production cost of the fuel supply system.
The use of the pressure control valve, the relief valve, the bypass path, and the check valve also results in a complicated structure of the fuel supply system and decreased mountability thereof in the vehicle.